A radio receiver (e.g., AM, FM, DAB, etc.) generally has two AGC (Automatic Gain Control) loops, one to control an intermediate frequency signal level and the other to control the signal level into a baseband demodulator.
FIG. 1a shows a functional block diagram for such a radio receiver comprising RF input IP1, voltage controlled amplifier A1, mixer M1, local oscillator input LO1, power detector PD1, RF gain control loop Loop1, channel filter F1, mixer M2, local oscillator input LO2, voltage controlled amplifier A2, an IF output OP1 to a baseband device, an IF Gain Control Loop Loop2, a link D1 between the loops, and a further input IP2 that provides a gain control voltage from the Baseband Device. More specifically, the radio receiver may consist of the following:                a) a low noise amplifier A1 whose gain is controlled by an internal voltage Vrfagc;        b) a first mixer M1 which “down converts” the RF input to a lower frequency, e.g., DC (0 Hz) for DAB or 124 kHz for FM;        c) a channel filter F1 which only allows the selected frequencies to pass through and attenuates any unwanted frequencies;        d) a second mixer M2 which “up converts” the output from the channel filter to a suitable frequency for the subsequent signal processing, e.g., to 2.048 MHz for DAB or 2.172 MHz for FM; and        e) a voltage controlled amplifier A2 to provide an output to a signal processor/demodulator (not shown; e.g., baseband signal processor programmed to demodulate) and controlled by a second internal voltage Vifagc.        
The link between the RF and IF control loops is generally provided to limit the reduction in baseband gain that occurs without also reducing the RF gain. Preferably, both amplifiers thus operate in the intended, preferably linear, regions of their voltage-controlled gain characteristics over as wide a range as possible, and/or so that the amplifier A1 does not reach its maximum gain under normal operation. The link is shown in FIG. 1a using a conventional diode symbol illustratively to indicate a digital diode, i.e., circuitry having a programmable voltage limit as compared to the non-programmable limit provided by the forward bias voltage of a conventional p-n diode.
A Received Signal Strength Indicator (RSSI) may be implemented on the basis of detecting the control voltage to the second amplifier (Vifagc). In order to reduce the effects of interference on RSSI however, the receiver may have a “tracking” filter at the RF input IP1 prior to the low noise amplifier A1. The purpose of this filter is to pass the wanted signal and reject any interfering signal. (The filter is called a “tracking” filter because it “tracks” the wanted signal frequency).
Such a tracking filter is however a low order filter and more effective at removing interfering signals that are at a frequency far away from the wanted signal.
Consequently, the field of radio receivers continues to provide a need to more reliably indicate quality of the received signal, preferably to more accurately indicate the signal strength of the wanted signal being received. Thus, it is desired to allow the presence an interferer to be detected and compensated for, in particular to provide an improved “Received Signal Strength Indicator” (RSSI) function. Similarly, there is a need in the field of radio receivers to indicate when interference may be present, preferably indicating the power of the interference.